This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars.
KEYWORDS: Nulling interferometry, Planets, Interferometers, Space operations, Mirrors, Robots, Control systems, Stars, Space telescopes, Interferometry
The last decade has seen great advances in interferometric nulling technology, propelled at first by the SIM and KECK
nulling programs and then by the Terrestrial Planet Finder Interferometer (TPF-I). In the infrared at N-band (using a CO2
laser at 10.6 micron wavelength) the first million to one nulls were reported on a KECK testbed in 2003. For TPF-I,
nulls needed to be both deep and broadband, and a suite of testbeds was designed and built to study all aspects of
achromatic nulling and system implementation, including formation flying technology. Also, observatory designs were
drawn up and studied against performance models. Modeling revealed that natural variations in the alignment and
control of the optical system produced an "instability noise" signal and this realization eventually led to a redesign of the
layout to a rectangular formation. The complexity of the early TPF-I spacecraft design was mitigated by the infusion of
ideas from Europe and produced the current X-Array design which utilizes simple reflectors to form the apertures
together with a stretched three dimensional formation geometry. This paper summarizes the main achievements of the
infrared nulling technology program including the development of adaptive nulling for broadband performance and the
demonstration of starlight suppression by 100 million to one.
Arrays of single mode fibers can be used to form segmented pupils of almost arbitrary geometry. Such pupil arrays can
be used both for interferometric imaging, for example by non-redundant aperture masking or in direct imaging systems
such as the phased array coronagraph. Achieving control over the optical coupling, phase and dispersion for fiber arrays
of reasonable size is a technological challenge. Progress has been made using a monolithic block of single mode fibers,
lens arrays and masks, and mirror arrays. On one testbed, arrays of up to 37 beamlets are being combined to form a
single image. On a second testbed, control of dispersion between fibers of slightly different length is being evaluated.
The combination of the techniques being demonstrated has a range of potential uses in astronomy. In this paper we
discuss the initial testbed results.
Single-mode optical fibers are playing an increasing role in astronomical interferometry, e.g., in high-accuracy visibility
measurements and in nulling interferometry. However, such observing modes typically involve only small numbers of
fibers. On the other hand, some recently proposed observing techniques call for arrays of single mode fibers coupled to
arrays of sub-apertures within a large telescope pupil. The concepts include pupil-masked visibility measurements (non-redundant
masking), pupil-sheared nulling interferometry, and coronagraphic imaging using a fiber-linked phased-array
of small optical telescopes. The latter arrangement may also be relevant to optical communications. Here we provide an
overview of a number of recent novel applications of single-mode fibers and single-mode fiber arrays.
The air quality of any manned spacecraft needs to be continuously monitored in order to safeguard the health of the
crew. Air quality monitoring grows in importance as mission duration increases. Due to the small size, low power draw,
and performance reliability, semiconductor laser-based instruments are viable candidates for this purpose. Achieving a
minimum instrument size requires lasers with emission wavelength coinciding with the absorption of the fundamental
absorption lines of the target gases, which are mostly in the 3.0-5.0 μm wavelength range. In this paper we report on our
progress developing high wall plug efficiency type-I quantum-well GaSb-based diode lasers operating at room
temperatures in the spectral region near 3.0-3.5 μm and quantum cascade (QC) lasers in the 4.0-5.0 μm range. These
lasers will enable the development of miniature, low-power laser spectrometers for environmental monitoring of the
spacecraft.
This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer
(TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like
planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress
with each of the testbeds is summarized.
Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can
help achieve deeper interferometric nulls. Silver halide fibers of composition AgClxBr1-x(0
This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer
(TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like
planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress
with each of the testbeds is summarized. The current interferometer architecture, design trades, and the viability
of possible reduced-scope mission concepts are also presented.
Spatial filtering is necessary to achieve deep nulls in optical interferometer and single mode infrared fibers can serve as
spatial filters. The filtering function is based on the ability of these devices to perform the mode-cleaning function: only
the component of the input field that is coupled to the single bound (fundamental) mode of the device propagates to the
output without substantial loss. In practical fiber devices, there are leakage channels that cause light not coupled into
the fundamental mode to propagate to the output. These include propagation through the fiber cladding and by means
of a leaky mode. We propose a technique for measuring the magnitude of this leakage and apply it to infrared fibers
made at the Naval Research Laboratory and at Tel Aviv University. All measurements are performed at 10.5 μm
wavelength.
We have devised a hybridization scheme that, given suitable Fabri-Perot (F-P) gain medium, allows us to fabricate small, mechanically robust single frequency lasers in a wide spectral range, limited only by the transparency of the SiON material. In this report we discuss device fabrication and present characteristics of a laser emitting in red spectral range. The laser operates at or near room temperature under continuous wave excitation and emits 5 mW of power in single mode with 40 dB side mode suppression.
We have built a miniature illuminator for Laser Doppler velocimeter on micromachined silicon optical bench utilizing a novel optical scheme. We used two intersecting coherent beams from the two opposing facets of semiconductor laser die to form a standing interference pattern needed for the particle detection and velocity measurement. Such devices are of interest to NASA for investigating wind patterns and dust loading on planets with atmosphere. They have been applied to problems where the liquid or gas flux must be characterized without disturbing the flow. In addition, the small probe volume makes possible local flow characterization and profiling. The device fabrication, and the results of the fringe characterization and velocity measurements are presented and discussed.
This paper describes the design of infrared filters using methods drawn from microwave and millimeter wave filters. Special note is made of approximations made in the infrared design, and ways to improve upon these approximations. Results from the design, manufacture and test of linear wedge filters built using microlithographic techniques and used in spectral imaging applications will be presented.
Linear variable filters have found increasing applicability in spectrally selective optical instruments. They serve as moderate resolution spectral discriminators in astronomical instruments and in reconnaissance equipment. They perform extremely well as 'sliding out-of-band blocking filters' when used in conjunction with grating spectrometers.
A new technology for infrared optical filters is presented. We have produced a band pass filter consisting of a thin (12.4 micrometers ) Si wafer with cross-shaped metal patterns deposited on both sides. The crosses, with 6 micrometers arms, have been formed by direct-write electron- beam lithography on 1000 angstroms Al film. The filter is mounted on a 0.2 mm thick Si frame with 0.25 cm2 window, but it also can be bonded to detectors so that the filter and the detector temperature are the same. Due to the high index of refractive in Si, this filter is more tolerant to converging beams than interferometric filters made of metal meshes stretched in air that have been reported previously. When placed into an F/3 converging beam, the filter has a bandpass characteristic centered at 70 micrometers wavelength with width at half maximum (delta) (lambda) /(lambda) equals 7%. The transmission maximum is 44%. At present the band pass is limited by the non-uniformity of the available substrates. The out-of-band rejection need improving which can be achieved using additional non-interferometric filters with wider band pass made with similar technology. Another way of improving the performance is integrating the metal patterns onto the detector which will make it frequency selective. Extending the interference filter technology to shorter wavelengths is difficult due to the onset of absorption in the metal layers.
SiGe/Si heterojunction internal photoemission (HIP) long wavelength infrared (LWIR) detectors have been fabricated by MBE. The SiGe/Si HIP detector offers a tailorable spectral response in the long wavelength infrared regime by varying the SiGe/Si heterojunction barrier. Degenerately doped p(+) SiGe layers were grown using elemental boron, as the dopant source allows a low growth temperature. Good crystalline quality was achieved for boron-doped SiGe due to the reduced growth temperature. The dark current density of the boron-doped HIP detectors was found to be thermionic emission limited. HIP detectors with a 0.066 eV were fabricated and characterized using activation energy analysis, corresponding to a 18 micron cutoff wavelength. Photoresponse of the detectors at wavelengths ranging from 2 to 12 microns has been characterized with corresponding quantum efficiencies of 5 - 0.1 percent.
In order to gain information about the band offset in the strained layer
InGa1As/GaAs system we have investigated photorefleCtanCe (PR) from two
single quantum wel 1 samples at 300 K and 77 K. Our samples have we 1 1 width
L= 110 A (sample 1) and L = 107 A (sample 2) with In Composition x = 0. 11
(sample 1 ) and x = 0. 19 (sample 2 ) . We have observed a number of intersubband
transitions in the spectra of both samples. By studying the polarization
dependence of the PR at 300 K using an internal reflection mode we have
identified spectral features Corresponding to light and heavy hole to
conduction subband transitions. Good agreement between our experimental
results and an envelope function calculation (including strain) is obtained
for conduction band offset Q = 0.45 0.07 (sample 1) and Q = 0.67 0.07
(sample 2). These values comply with the compositional dependence of
proposed by Joyce et al [Phys.Rev. B 38, 10978 (1988)1.
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